There are many products available that employ lithium disilicate glass ceramic covered by several U.S. patents. Some of these patents claim a process for the preparation of shaped translucent lithium disilicate glass ceramic products from a mixture of basic components (SiO2, Al2O3, K2O, Li2O, plus pigments and fluorescent oxides). The thermodynamic solid-liquid equilibrium of the system consisting of lithium oxide (Li2O) and silicon dioxide (SiO2) has been extensively studied even before that material was used as a dental ceramic (1-3, 5-6).
For those skilled in the art this experimental solid-liquid equilibrium can explain with an extraordinary simplicity how different glass ceramics can be obtained using the same two components when they are combined in different proportions. The same solid-liquid equilibrium shows what type of stable crystal is produced as a final product of crystallization when a specific mix composition of the two components are blended, melted, and crystallized to achieve the final product.
The crystallographic data for intermediate crystal compounds in the Li2O—SiO2 system is given by the Landolt-Bornstein tables. The following are the types of crystal compositions possible in the Li2O—SiO2 system: Li8SiO6, Li4SiO4 or lithium ortho silicate monoclinic and orthorhombic; Li6Si2O7, Li2SiO3 or lithium silicate; Li2Si2O5 or lithium disilicate monoclinic and orthorhombic; and Li2Si3O7 lithium trisilicate.
Thus when the silicon dioxide to lithium oxide molar ratio (SiO2/Li2O) is greater than or equal to two, meaning two moles of SiO2 are mixed with one mol of Li2O, the crystallized glass ceramic product will be mainly lithium disilicate (SiO2/Li2O). This molar ratio of two is equivalent to a molar composition of lithium oxide in the mixture of 33% (67% as SiO2). When the same molar relationship is below 2.0, (e.g. 1.7) only lithium silicate crystals are produced (Li2O.SiO2). The lithium oxide molar composition for a ratio of 1.7 is equivalent to 37% molar (63% SiO2). The type of resulting crystal due to the specific composition ratios gives to the glass ceramic its own distinguishable chemical and physical properties. Surprisingly, the same behavior is obtained if these two main components (silicon dioxide and lithium oxide) maintain their molar ratio below two even if they are mixed with other oxides as additives and modifiers. The other common oxides mixed are aluminum oxide, potassium oxide, calcium oxide, zirconium oxide and coloring oxides that are incorporated into the glass matrix and give the glass ceramic its final color and translucency.
Due to the final composition of this invention using a molar ratio of SiO2/Li2O between 1.7 to 1.9, the only phase present is lithium silicate, instead of lithium disilicate, as a main constituent of the glass ceramic as a final product. For instance, a glass ceramic with a molar ratio of silicon dioxide to lithium oxide greater than or equal to two plus additional oxides will produce, after full crystallization a lithium disilicate glass ceramic with a melting temperature of 920° C. and a linear thermal coefficient of expansion of 10.5×10−6/° C. as a final product and composition. In addition, during the production of this type of glass ceramic the cast material is subjected to at least three different heat treatments: an annealing cycle for eliminating accumulated stresses, a nucleation cycle for the formation of lithium metasilicate or unstable lithium silicate, and finally a third thermal cycle to convert the unstable lithium silicate or metasilicate into a stable lithium disilicate. This is clearly shown in the following US patents:
Examples of those types of glass ceramics are claimed in Barret et al in U.S. Pat. No. 4,189,325 which discloses a lithium silicate glass ceramic where the raw materials are blended, melted at 1315° C. and held for 24 hours for homogenization, fritted and crushed, melted again and cast into preheated molds. They disclose a composition of silicon dioxide to lithium oxide molar ratio of two, producing a dental ceramic composed of lithium disilicate.
U.S. Pat. No. 4,480,044 to McAlinn discloses a glass ceramic formulation where the lithium silicate glass ceramic in their intermediate process stage has a thermal expansion of 13×10−6/° C. and the lithium disilicate has a thermal expansion of 11.4×10−6/° C. They disclose a machinable lithium disilicate glass ceramic with a percentage of silicon dioxide of 79.8%.
U.S. Pat. No. 4,515,634 to Wu et al discloses a castable glass ceramic composition useful as a dental restorative material. The components are blended and melted at 1400 to 1450° C., then quenched in water, dried, milled to a powder, and melted again at 1400° C. for 4 hours. Then the melt is cast into copper molds and transferred to the annealing process. The castable glass ceramic of the invention is lithium disilicate with a silicon dioxide to lithium oxide molar ratio of two, equivalent to silicon dioxide weight composition of 65%-74.7% and lithium oxide weight composition of 14.8-16.4%.
U.S. Pat. No. 5,219,799 to Beall et al discloses a lithium disilicate glass ceramic with silicon dioxide weight composition of 65%-80% and lithium oxide compositions of 8.0-19.0%. The blended raw materials are melted at 1450° C. for 16 hours and then poured into steel molds and annealed at 450° C.
U.S. Pat. No. 5,744,208 to Beall et al describes a lithium disilicate glass ceramic with silicon dioxide weight composition of 75%-95% and lithium oxide weight composition 3-15%. The raw materials are blended, and then melted in the range of 1450-1600° C. for about 6-10 hours. The glass is then poured into steel molds. The glass is then annealed, nucleated and crystallized to produce lithium disilicate glass ceramic in the range of 500° C. to 850° C.
U.S. Pat. No. 5,968,856 to Scheweiger et al discloses a lithium disilicate glass ceramic with compositions of silicon dioxide weight between 57%-80% and lithium oxide composition 11-19%. The components are blended and melted at 1500° C. for one hour and then quenched, dried, milled, dry pressed and sintered to form blanks. The composition requires the addition of lanthanum oxide to improve the flow properties, control the crystal growth and eliminate the strong reaction of the material with the investment material used.
U.S. Pat. No. 6,514,893 to Scheweiger et al discloses a lithium disilicate glass ceramic with silicon dioxide composition of 57%-75% weight and lithium oxide composition 13-19% weight and also containing lanthanum oxide. The components are blended and fused into granulates and comminuted to a powder. Coloring oxides are then added, and the ceramic is pressed and heat treated.
U.S. Pat. No. 6,455,451 to Brodkin et al discloses a lithium disilicate glass ceramic with silicon dioxide composition of 62%-85% weight and lithium oxide composition 8-19% weight. They disclose a method of making the lithium disilicate by melting the components at 1200 to 1600° C., followed by a quench, drying, and a heat treatment to form the glass ceramic, followed by comminuting to a powder, compacting and sintering to a blank and pressing to form the restoration.
U.S. Pat. No. 6,517,623 to Brodkin et al discloses a lithium disilicate glass ceramic pressable where the components are melted in the range of 1200 to 1600° C., quenched, heat treated, comminuting the glass ceramic to a powder, and then compacting the powder to a starting blank before sintering the blank or the restoration.
U.S. Pat. No. 6,606,884 to Scheweiger et al describes a lithium disilicate glass ceramic where the components are mixed and melted at 1200 to 1650° C., followed by pouring the glass into water, milling and compacting, and placing the blank in a heat treatment to sinter.
U.S. Pat. No. 6,802,894 to Brodkin et al shows a lithium disilicate glass ceramic with a silicon dioxide weight composition of 62%-85% and lithium oxide weight composition 8-19%. The components are mixed, melted at 1200 to 1600° C., and cast. The resulting glass is annealed at a range of 300 to 600° C., followed by subjecting the glass to a heat treatment from 400 to 1100° C.
U.S. Pat. No. 6,818,573 to Petticrew discloses a lithium disilicate glass ceramic with a silicon dioxide composition of 60%-80% weight and lithium oxide composition of 8-17% weight. The components are blended, melted, quenched, heat treated, milled to a powder, dry pressed, and hot pressed into the desired restoration.
U.S. Pat. No. 7,316,740 to Scheweiger et al claims a lithium silicate glass ceramic with silicon dioxide weight compositions of 64 to 73% and lithium oxide weight compositions of 13 to 17%. The lithium disilicate final product is demonstrated by means of a XRD pattern (FIG. 6) and DSC phase transformation curve from lithium metasilicate to lithium disilicate (FIG. 2). The DSC diagram shows the change in energy from the stage of metasilicate to disilicate, which is only necessary if lithium disilicate is desired to be the crystal phase used as a final product.
U.S. Pat. No. 7,452,836 to Apel et al discloses a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight producing lithium disilicate as a final product. They also describe a glass ceramic with a molar ratio of silicon dioxide to lithium oxide of at least 2.3.
U.S. Pat. No. 7,867,930 to Apel et al shows a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight producing lithium di silicate as a final product.
U.S. Pat. No. 7,871,948 to Apel et al describes a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight, producing lithium disilicate as a final product. The glass of the starting material is subjected to an initial heat treatment form lithium metasilicate or unstable lithium silicate and then goes through a second heat treatment to convert the lithium metasilicate to a lithium disilicate.
U.S. Pat. No. 7,867,931 to Apel et al discloses a lithium silicate glass ceramic with silicon dioxide composition of 64%-75% weight and lithium oxide composition of 13-17% weight producing lithium disilicate as the final product. They also describe a glass ceramic with a molar ratio of silicon dioxide to lithium oxide in the range of 2.3 to 2.5.
U.S. Pat. No. 8,042,358 to Schweiger et al discloses a lithium silicate glass ceramic with silicon dioxide composition of 65%-70% weight and lithium oxide composition of 14-16% weight producing lithium disilicate as the final product. In their specific process the raw materials such as carbonates, oxides and phosphates are prepared and melted in the range of 1300-1600° C. for 2 to 10 hours. They explain that in order to obtain a particularly high degree of homogeneity the glass melt obtained may be poured into water to form glass granulates and the glass granulates obtained are melted again.
For those skilled in the art it is understandable that the lithium oxide-silicon dioxide system has been extensively studied and several patents for dental glass ceramics have been granted in the last few years. However all the research so far falls in the range where lithium disilicate is formed as a final product and none of the references above disclose lithium silicate glass ceramic as a final product. For those skilled in the art it is evident that the type of crystal produced depends exclusively on the molar ratio of silicon dioxide to lithium oxide in the glass ceramic and not the additives or modifiers added to the mixture. This molar ratio controls the type of crystal formed in the final composition and furthermore give its name to the final glass ceramic.
Most of the existing patents in the dental field use the same basic components. The present invention uses germanium dioxide as a fundamental part of the formula. This oxide is broadly used in glass preparation for its good optical properties. The oxide has been well studied and has positive effects compared to common silicon glasses. It has been found that the addition of germanium oxide produces a melt with low viscosity, which facilitates the castability of the process and increases the thermal expansion and the refractive index of the resulting lithium silicate glass ceramic. More importantly, the addition of germanium dioxide increases the final density of the glass resulting in higher values of flexural strength than the lithium disilicate glasses free of germanium dioxide. U.S. Patent Application Publication No. 2004/0197738 to Ban et al. discloses a process to make dental frame of zirconium-yttrium sintered ceramics and they describe dental porcelain with germanium oxide as a joint component different than the zirconium yttrium oxide frame. However germanium oxide is not used as a component of the framework ceramic network. It is used only in formulation of the ceramic joint and is just a part of a series of other oxides that can be joined to the framework material.
Due to the low silicon dioxide to lithium oxide molar ratio of 1.7 of the present invention, equivalent to 37% molar of lithium oxide (63% silicon dioxide) the ceramic has a lower melting point compared to the glass ceramic of the prior art. In addition, this new glass ceramic contains the lowest silicon dioxide weight percent compared to all of the noted prior art. Therefore, due to this specific composition of lithium oxide in the mixture, the type of resulting crystal after crystallization (lithium silicate) gives to the glass ceramic its own chemical and physical properties, which makes it completely distinguishable from the prior glass ceramics listed above. Due to this distinguishable composition, the present glass ceramic has a lower melting temperature and can be made even lower with the addition of germanium oxide. Germanium oxide replaces silicon dioxide in the glass network, causing it to have a negative effect on the resulting melting point compared to a glass ceramic containing only silicon dioxide. Thus the processing and optimal melting temperature is in the range of 1100° C. to 1200° C. instead of 1200° C. to 1650° C. of the U.S. patents cited above and specifically compared to U.S. Pat. No. 6,514,893 to Schweiger et al. The glass ceramics listed in the prior art patents cannot be cast in the range of 1100° C. to 1200° C. because they are too viscous due to their high silicon dioxide content, therefore the processes disclosed in prior art patents with higher melting temperatures should be used. The present process will result in a more economical production because the energy employed for melting the glass is considerably lower and there are lower energy losses by radiation compared to the Schweiger process.
In addition to having a process with lower energy consumption, another significant improvement of the inventive process is related to the mixing and reaction of the components. In all of the previously cited prior art patents, the mix of the components is blended and melted at 1400 to 1650° C. and then cast or quenched in water. The quenched glass powder is dried, milled, and melted again in order to improve the homogeneity and the quality of the product. Surprisingly, it was found that the first melting and casting process can be avoided if one performs a calcination process on the mixture of raw materials to a temperature in the range of 700 to 800° C. without melting the components. At this stage, all the raw materials in the form of salts (like lithium carbonate as the source of lithium oxide, calcium carbonate as the source of calcium oxide, and di-ammonium phosphate as the source of phosphorous oxide) are decomposed, eliminating gases such as carbon dioxide and ammonia, producing a ceramic powder free of gases. After cooling down, the calcined mix is milled again, producing a homogeneous powder with a very small particle size. The final step is melting and casting in the range of 1100° C. to 1200° C., resulting in a homogeneity of all the components. In addition, by eliminating the gases during the calcination process, the glass cast becomes bubble free, making this a significant advantage over the processes described in the prior art.
The present invention is also unique compared to those listed in the prior art due to its composition. The use of a low melting temperature is only possible with the present glass ceramic because of the low content of silicon dioxide and the high content of lithium oxide. This translates to a molar oxide ratio (SiO2/Li2O) below 2.0, (i.e., 1.7) in which only lithium silicate crystals are produced (SiO2—Li2O). In addition to the composition, the present invention implements a process for a glass ceramic that produces a homogeneous product and that can be used only with the specific formulation. This process cannot be used with the other listed glass ceramics due to the lower operating temperatures.
The present invention emphasizes that in the inventive glass ceramic the silicon dioxide and lithium oxide molar ratio content (SiO2/Li2O) is less than 2, specifically the oxide molar ratio is preferably about 1.7. This is specifically equivalent to 63% molar of silicon dioxide and 37% molar of lithium oxide, and specifically equivalent in the overall formulation of about 56% weight percent of all of the glass ceramic as silicon dioxide and 16.0% weight percent as lithium oxide and the remaining 28% composed of the oxide additives and modifiers. In all of the glass ceramic, only lithium silicate (Li2O—SiO2) crystals are produced as the final crystal phase product. During the heating process of the glass, the first crystals formed are stable lithium silicate and they remain stable through the end of the growing process. This means that there is no need for a third thermal process for producing the final crystal of lithium silicate making this an additional characteristic unique to the present invention. This new ceramic has softening temperature of about 700 to 800° C. and a linear thermal coefficient of expansion of about 12 to 12.5×10−6/° C. as a final product and a composition yielding completely different chemical and physical properties compared to the prior art. This is easily demonstrated in commonly assigned U.S. patent application Ser. No. 12/592,825, paragraph [0012], FIG. 1 showing a XRD pattern diffraction where only lithium silicate crystals are present in the final product and paragraph [0013] where the glass ceramic has a percentage linear change of 0.55% measured at 500° C. and an equivalent coefficient of thermal expansion of 11.5×10−6/° C.
The following is a list of non-patent references noted herein:    1. MARCUS P. BOROM et al, Strength And Microstructure In Lithium Disilicate Glass Ceramics, J. Am. Ceram. Soc., September-October 1975, Vol. 58, No. 9-10, pp 385-391. The authors prepare lithium disilicate glass ceramics and measured the differences between the thermal expansion of the lithium disilicate with a value of 13×10−6/° C. and lithium silicate with a value of 11.4×10−6/° C. After the heat treatment above 800° C. the only phase present is lithium disilicate for a glass ceramic composition of 71.8% of silicon dioxide and 12.6 of lithium oxide.    2. R. A. EPPLER, Glass Formation And Recrystallization In The Lithium Metasilicate Region Of The System Li2O—Al2O3—SiO2, J. Am. Ceram. Soc., February 1963, Vol. 46, No. 2, pp 97-101.    3. F. A. HUMMEL, Thermal Expansion Properties Of Some Synthetic Lithia Minerals, J. Am. Ceram. Soc., August 1951, Vol. 34, No. 8, pp 235-239.    4. LANDOLT-BORNSTEIN (LB), Group IV Physical Chemistry—Phase Equilibria, Crystallographic And Thermodynamic Data For Intermediate Compounds in the Li2O—SiO2 System.    5. S. CLAUS et al, Phase Equilibria In The Li4SiO4—Li2SiO3 Region Of The Pseudobinary Li2O—SiO2, Journal of Nuclear Materials, Vol. 230, Issue 1, May 1996, pp 8-11.    6. HERMAN F. SHERMER, Thermal Expansion Of Binary Alkali Silicate Glasses, Journal of Research of the National Bureau of Standards, Vol. 57, No. 2, August 1956. The author prepares lithium silicate glasses with silicon oxide and lithium oxide molar ratio below 2.0 being lithium silicate with thermal expansion between 12 and 14.77×10−6/° C. There is no lithium disilicate using this chemical molar composition.